Welcome to HCM
We understand your concerns
I think your diagnosis is just superficial. Masturbaation cnnot be a proper reason for dark patches on penis. We all rub our hands and feet often. Any dark patches come?
Secondly you need to consult dermatologist
to find out what are those dark patches if it is itching, sore, troublesome etc. Dermatologist cn help you. Till then no anxiety
Wish you speedy recovery